1. Field of the Invention
The present invention generally relates to improved apparatus for dispersing gas bubbles throughout a liquid body.
2. State of the Art
It is well known to distribute gas bubbles in a liquid body in order to accomplish, for example, solid-liquid or liquid-liquid separation by flotation. Such flotation techniques are commonly used for separating and concentrating valuable minerals and chemicals, for removing particulates from liquid bodies and for separating various liquids. A typical flotation process in the mineral beneficiation art, for example, includes the steps of conditioning an aqueous pulp or slurry of crushed ore with a chemical flotation aid and then dispersing air bubbles within the pulp to produce a surface froth relatively rich in the desired mineral. In the field of oil production, similar flotation processes are frequently used to separate crude oil from water prior to the reinjection of the water into a well or prior to surface disposal of the water. In flotation processes in general, it is important to maximize contact between the froth-producing gas bubbles and the materials which are to be floated and, at the same time, to maintain the surface of the liquid body fairly quiescent so that the froth is not agitated so much as the cause the floated materials to separate from the gas bubbles to which they have become attached.
Dispersed gas flotation, as distinguished from dissolved gas flotation, achieves physical separation of a contaminating substance from a body of primary liquid by effecting contact between the contaminating substance, which may be either solid particles or a second liquid, and gas bubbles without first dissolving the gas in the primary liquid. Having achieved contact, the contaminating materials attach to the gas bubbles and rise buoyantly to the surface of the primary liquid as a froth which can be subsequently removed, as by skimming. In dispersed gas flotation systems, it is important to achieve small gas bubbles (i.e. high surface-to-volume ratio), good mixing to assure high gas-particle contact probability, minimum short circuiting of the primary liquid, and a highly concentrated contaminant level in the removal stream.
Conventional dispersed gas flotation systems, which utilize mechanical impellers in flotation cells to ingest gas into liquids, have inherent features which preclude their application to many areas, most notably the treatment of wastewater in municipal plants and in pulp and paper mills. Attempts to apply mechanical-type gas flotation devices in such areas have failed because of the inherently high degree of fluid turbulence produced by the impellers within the separation zone of the flotation cells and the necessity for baffles in the mixing zones of the cells. In pulp and paper applications, for example, high fluid turbulence will break up the relatively weak floc in the wastewater. In municipal waste treatment or when treating wastewater from meat-packing plants, as another example, the mechanical elements and baffles in conventional gas flotation systems foul due to the presence of "stringy-type" solids.
An example of a mechanical-type flotation machine, which includes a rotatable impeller to aspirate gas into a liquid body in a vessel and to agitate the liquid to distribute the gas is shown in U.S. Pat. No. 3,491,880 to W. H. Reck. Flotation machines which utilize one or more gas injection nozzles in combination with a baffle arrangement to distribute gas within a liquid body are shown in U.S. Pat. Nos. 2,008,624; 3,371,779; and 3,446,353. Dissolved air flotation machines, wherein air is dissolved under pressure in a stream of liquid and then the liquid from the stream is injected into a larger body at reduced pressure to liberate bubbles to accomplish flotation, are shown in U.S. Pat. Nos. 2,759,607 and 3,418,236.